Various compounds have been known as 1,5-pentanedial having an alkyl group at the 3-position. For example, 3-methylglutaraldehyde (3-methyl-1,5-pentanedial, which is hereinafter abbreviated as MGL) is a compound that is useful as a curing agent for a photosensitive material, a leather tanning agent, and a synthetic intermediate (see, for example, PTLs 1 to 3). The known production methods of MGL include a method of hydrolyzing pyranyl ether obtained through Diels-Alder reaction of crotonaldehyde and methyl vinyl ether (see NPLs 1 and 2).